The demands for greater switching speed and circuit performance have seen the advent of new dielectric materials (dielectric constant of <3) and metals to reduce the RC delay constant in circuits. The metal of choice, which is copper, has added several challenges to the process integration scheme. For aluminum interconnects, the metal patterning was performed by reactive ion etching (RIE) of the aluminum followed by dielectric deposition. With copper, the dielectric film is first deposited and etched to form vias and trenches followed by the deposition of copper in those etched features. The excess copper is then removed using chemical mechanical polishing (CMP) to planarize the surface for subsequent layers of film. This method of forming copper interconnects for the back-end-of-line (BEOL) is known as the Dual Damascene process.
Following the dielectric etch to form the vias and trenches, a large amount of fluoropolymeric residue is left both on the surface of the wafer and on the inside of features as seen in FIG. 1. These residues are generated during the etching process, partly for sidewall passivation during anisotropic etching. The etch residue has to be cleaned prior to the deposition of the successive film layers: the copper barrier Ta/TaN film, copper seed layer, and finally the electrochemical filling of the features with copper in the Damascene process.
The dimensions of the features used in the interconnects at the BEOL are currently around 0.13 μm. For cryogenic cleaning to work effectively in removing the sidewall residues from inside the features, as shown in FIG. 1, the cryogenic particles must be less than 0.13 μm in size. As well, these particles must arrive at the surface of the wafer with enough velocity to impart the momentum transfer required to dislodge the sidewall residue.
There are three mechanisms by which surface cleaning is done: 1) momentum transfer by cryogenic particles to overcome the force of adhesion of slurry particles to the wafer surface, 2) drag force of the cleaning gases to remove the dislodged particles off the surface of the wafer, and 3) the dissolution of organic contaminants by liquid formed at the interface of the cryogenic particle and the wafer surface.
In CO2 cryogenic cleaning, liquid CO2 at a pressure of about 850 psi from a purified source is made to expand through the orifice of a specially designed nozzle intended to make the expansion a constant enthalpy process. The expansion of liquid CO2 through the nozzle creates solid and gaseous CO2 in a highly directional and focused stream. Due to the gas flow over the wafer surface, a boundary layer is formed. The CO2 cryogenic particles must travel through the boundary layer to arrive at the wafer surface and at the contaminant particle to be removed. During the flight through the boundary layer, their velocity decreases due to the drag force on them by the gaseous CO2 in the boundary layer. Assuming the thickness of the boundary layer to be h, a snow particle must enter the layer with a normal component of velocity equal to at least h/t where t is the time taken to cross the boundary layer and arrive at the wafer surface. The relaxation time of the particle crossing the boundary layer is given in equation (1) as the following:                     τ        =                              2            ⁢                          a              2                        ⁢                          ρ              p                        ⁢                          C              c                                            9            η                                              (        1        )            where:
a is the particle radius
ρp is the particle density
η is the viscosity of the gas
Cc is the Cunningham slip correction factor given as in equation (2)Cc=1+1.246(λ/a)+0.42(λ/a)exp[−0.87(a/λ)]  (2)where λ is the mean free path of gas molecules. Since the CO2 cryogenic cleaning is conducted at atmospheric pressure, the Cunningham slip correction factor becomes equal to 1 in equation (1) for cryogenic particles larger than 0.1 μm in size.
Thus, for CO2 snow particles to have sufficient momentum to remove foreign material from the wafer surface and from inside the features, the time to cross the boundary layer must be less than the relaxation time, in which case they will arrive at the surface with greater than 36% of the initial velocity. Equation 1 shows that the relaxation time decreases with particle size. Therefore, the smaller-sized particles will not be able to arrive at the wafer surface with sufficient velocity to effectively clean the inside walls of the submicron vias and trenches.
The prior art processes generally use CO2 or argon cryogenic spray for removing foreign material from surfaces. As examples, see U.S. Pat. No. 5,931,721 entitled Aerosol Surface Processing; U.S. Pat. No. 6,036,581 entitled Substrate Cleaning Method and Apparatus: U.S. Pat. No. 5,853,962 entitled Photoresist and Redeposition Removal Using Carbon Dioxide Jet Spray; U.S. Pat. No. 6,203,406 entitled Aerosol Surface Processing; and U.S. Pat. No. 5,775,127 entitled High Dispersion Carbon Dioxide Snow Apparatus. In all of the above prior art patents, the foreign material is removed from a relatively planar surface by physical force involving momentum transfer to the contaminants. However, such cleaning methods are inadequate for features with high aspect ratios such as in vias and trenches in the back-end-of-line integrated device fabrication process where removal of small submicron particles and complex polymeric residues, as generated by dielectric etch processes, is required.
U.S. Pat. No. 6,332,470 entitled Aerosol Substrate Cleaner discloses the use of vapor only or vapor in conjunction with high pressure liquid droplets for cleaning semiconductor substrate. Unfortunately, the liquid impact does not have sufficient momentum transfer capability as solid CO2 and will therefore not be as effective in removing the smaller-sized particles. U.S. Pat. No. 5,908,510 entitled Residue Removal by Supercritical Fluids discloses the use of cryogenic aerosol in conjunction with supercritical fluid or liquid CO2. Since CO2 is a non-polar molecule, the solvation capability of polar foreign material is significantly reduced. Also, since the liquid or supercritical CO2 formation requires high pressure (greater than 75 psi for liquid and 1080 psi for supercritical), the equipment is expensive. U.S. Pat. No. 6,231,775 proposes the use of sulfur trioxide gas by itself or in combination with other gases for removing organic materials from substrates as in ashing. Such vapor phase cleaning is inadequate for removing cross-linked photoresist formed during the etching in a typical dual Damascene integration scheme using low k materials such as carbon doped oxides.
As such, there remains a need for the effective and efficient removal of homogeneous and inhomogeneous contaminants consisting of cross-linked and bulk photoresist, post-etch residues, and sub-micron sized particulates both from the surface of the wafer as well as from inside high aspect ratio features.